Catheters of various types and sizes have been used by physicians extensively. One use of the catheter is in providing regional anesthesia which produces profound analgesia with minimal physiologic alterations. When used at the start of an operation, regional anesthesia minimizes the total dosage of inhalation or intravenous anesthetic drugs required, hastens awakening, and permits early ambulation. When administered at the conclusion of surgery, regional anaesthesia produces post-operative analgesia with reduced risk of respiratory depression. Furthermore, certain types of pain are difficult to treat with systemic narcotics. For example, a bladder spasm following genitourinary surgery may be exacerbated by systemic opioids but is easily treated with a caudal epidural block. When prolonged analgesia is required, a catheter is inserted into the caudal or lumbar epidural space to provide intermittent or continuous injections of local anesthetics.
Caudal epidural anesthesia is notable for its simplicity, safety, and effectiveness and is one of the most frequently used regional anesthetic techniques for operations below the diaphragm in children.
When continuous pain relief is desired, the only equipment presently available is either a 19 or 20 gauge epidural catheter which is passed through either a 17 gauge Tuohy or an 18 gauge Crawford needle. Designed specifically for adults, these needles are approximately 3 1/2" long and have an outside diameter ranging from 0.050" to 0.059" along with an inside diameter ranging from 0.033" to 0.041". However, these needles are extraordinarily cumbersome to use in children, since the distance from the skin to the epidural space is only 10-15mm. Obviously, smaller needles and catheters are desirable.
The smallest presently offered epidural catheter is a 20 gauge continuous epidural catheter with an outside diameter of approximately 0.035". This catheter is constructed of a spiral wound stainless steel helix with a polymer plastic coating on the outside surface of the helix. The outside surface of the polymer coating is smooth, whereas the inside surface of the coating conforms to the outer surface of the spiral helix. The spiral helix is tightly wound with adjacent windings being in physical contact with each other. In effect, a spiral groove is formed in the outside surface of the helix in which the polymer coating conforms. As a result, the thickness of the polymer coating varies according to the contour of the outside surface of the helix. A problem with this nonuniform thickness coating occurs when the catheter is flexed or bent subjecting the polymer coating to forces that are more than sufficient to rupture or tear the polymer coating. Since the coating is not allowed to slide or move longitudinally over the outside surface of the helix, the polymer coating easily ruptures or tears when the catheter is flexed or bent. Not only is the polymer coating susceptible to being easily ruptured when flexed, the coating has fluid pressure limitations as well.
Another problem with this catheter is that the windings at the distal end of the spring wire helix are uncoated and have been expanded to permit dispersal of an injectate. A safety ribbon wire attached to the distal end prevents stretching of the helix. However, these uncoated and expanded distal windings are susceptible to the ingrowth of tissue, particularly with long-term placement. With tissue ingrowth, removal of the catheter causes trauma to the insertion site as well as possible injury to the dura.
Even with the failures of others, it is still desirable to reduce the outside diameter of a catheter to as small a value as is practical. Merely reducing the dimensions of existing catheters, however, introduces significant problems, one of them being that one must maintain a minimum fluid volume delivery rate.
Delivering a minimum level of fluid volume at an acceptable flow rate and pressure with a small diameter catheter is limited by the inside diameter of the catheter, the thickness of the catheter wall, and the pressure at which the fluid is delivered to the catheter. Calculating the fluid volume at a prescribed flow rate and pressure for a given inside diameter of a catheter is a straightforward matter. However, simply reducing the outside diameter of the catheter, while maintaining the inside diameter, decreases the thickness of the catheter wall and introduces a number of other concerns. These concerns include the susceptibility of the catheter to kinking when flexed or bent and the maximum pressure at which the fluid may be delivered without rupturing the catheter wall. As the thickness of the catheter wall decreases, susceptibility of the catheter to kinking increases. At a minimum, kinking of the catheter wall reduces the fluid volume delivery rate and, in many cases, causes a fluid stoppage and a rupture of the catheter wall with an accompanying loss of fluid. Furthermore, the reduced wall thickness must be capable of withstanding the delivery pressure without a rupture of the wall.
Flexibility of the catheter is increased with thinner wall thicknesses. However, the susceptibility of the catheter to kinking when flexed or bent also increases along with the probability of rupture due to kinking or high fluid pressures.
Minimum fluid volume delivery rates may be maintained with an increased delivery pressure when the inside and outside diameters of the catheter are decreased. However, an increase in delivery pressure must not cause injury to the tissue in the vicinity of a catheter. Delivery pressure is also limited by the connector attached at the proximal end of the catheter.
A number of small diameter catheters less than 0.033" have been attempted with one or more layers of polymer plastic material. However, the interrelationship between flexibility, kink-resistance, and fluid delivery pressure has prevented the introduction of an all plastic material catheter that is less than 0.033" in diameter and that is flexible, kink-resistant, and capable of delivering a volume of fluid at a rate which is acceptable by the medical community.